Barbaralane, homoaromaticity

We are, of course, aware of the strict Hiickel definition linking aromaticity to the presence of 4n + 2 rc-electrons, On the other hand, we note that the radical cations discussed below have SOMOs, which have one electron less than the Hiickel requirement and which show cyclic homoconjugation, even though the parent molecules assume alternative structures. In view of the precedence established for the radical anion (156) of bicyclo[6.1.0]nonatriene [416,417], in order to emphasize the dramatic difference between the structures of these radical cations and their precursors, and for reasons of convenience, we will refer to these species as homo- or bis-homoaromatic. The paradigm of these remarkable species is found in the barbaralane system. 

In contrast, CIDNP results indicate that the radical cations of barbaralane (157, X = C = 0) and semibullvalene (157, X = —) correspond to the elusive structure type with a single minimum [391, 424]. The spin density resides primarily on the termini (C-2,4,6,8) of the twin allyl moieties, whereas the remaining (internal) carbons of the 5 jr-electron perimeter have negative spin density. This spin density distribution reflects the coefficients of orbital 158, the HOMO of a bis-homoaromatic structure (Fig. 32) [424], More recently, ESR results have confirmed this assignment [392, 393],... 

Aromaticity and homoaromaticity of a parent barbaralane and a tetraphospha-barbaralane of C2v-symmetry were visualized by means of three-dimensional nucleus-independent chemical shift maps [90]. In combination with CPMD simulations the fiuxional character of tetraphosphabarbaralane was revealed and it was shown that the ionic motion at room temperature leaves the aromaticity in this case unchanged [90]. 

Childs. Cremer, and Elia maintain that there are several examples of neutral homoaromatics. They suggest that 1,2-dihydroborete (70) and some of its derivatives are neutral homoaromatics. This electron-deficient system is isoelectronic with the mono-homocyclopropenium cation (17). It has been investigated by both theory and experiment, and its classification as homoaromatic is justified. In addition to 70, Childs, Cremer, and Elia propose that certain semibullvalenes, barbaralanes, bridged an-nulenes, cycloheptatrienes, and norcaradienes are homoaromatic. They draw particular attention to the semibullvalenes 71-75 and consider that 74 and 75 are the best candidates for experimental work. 

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